Self propelled articulated boom aerial towers have gained considerable acceptance and popularity in the art in such applications as servicing and constructing electrical power lines, telephone lines, street light maintenance, etc. Experience has proven that many shortcomings exist in the control systems for these so-called "cherry-pickers".
The individual in the basket, at the end of the articulated upper boom, must have available a set of hydraulic controls which are intended to place the basket and occupant at a precise location in relation to the work performed. Since the item worked upon is a potentially dangerous power transmission line or the like, it is necessary to provide the operator with a precise, dependable control system. The control system should be, and is intended to be, capable of positioning the basket, and the occupant, in any one of three dimensions, i.e. vertically, horizontally and laterally. The difficulty with many of these controls has been the tendency to cause lurching, particularly at start-up and stop of the basket in any given direction responsively to actuation of the controls by the operator. The controls now available and in current use on "cherry-pickers", are generally deficient in that they do not provide for "feathering" or a fine control whereby the operator can gradually, and with a sure touch, bring the basket to whatever position is desired and avoid lurching or vibrating the basket as it moves from one position to the next by periodically stopping and starting.
It has been further found, that a "cherry-picker" must have as a safety precaution to the occupant in the basket, a ground level means for returning the basket through controls which are located at ground level so that in the event the basket occupant controls are either inoperative or the occupant is unable to operate the controls for one reason or another, the basket can be safely returned to ground level entirely by controls at the ground level.
Therefore, a dual control system is required in which there are two control systems; one in the basket accessible to the operator therein, and a second control system at ground level and available to a second operator; both such control systems being independently operable to effect energization of means for moving the basket in its vertical, horizontal, and lateral movements.
It is particularly important that the ground control system can in no way hamper the control system in the basket. Similarly, the control system in the basket should not be dependent upon movement of the ground control. Thus, neither control system interferes with the other in its normal independent operation.
One problem of control systems in general is known as "hysteresis", a condition in which a system continues to operate even after the demand for such movement is terminated. A characteristic of the present invention is that the system not only admits "feathering", i.e. fine control, but is completely free of hysteresis or unintended movements of the basket.